The invention relates generally to motor or generator components and, more particularly, to an improved flywheel arrangement for storing rotational energy.
Modern flywheels are preferably made from composite fiber materials and can be driven up to extremely high peripheral speeds, e.g. 800 to 1000 meters per second. Appropriate high-speed electrical machines, which may operate as motors and/or generators, are used for coupling-in and coupling-out the stored rotational energy.
The stress induced radial displacements in typical modern flywheels are considerable and are much larger than the permitted radial displacements of the rotor yoke in the electrical machine connected to the flywheel. Thus, the permissible peripheral speed of the inertial ring is always substantially greater than that of a machine rotor using a conventional stack of laminations. If one takes into account that the high stability of the composite fiber material can only be utilized with rings having a radius ratio of inner radius Ri to outer radius Ro wherein Ri/Ro is greater than 0.7, then this leads to compatibility problems. Three primary flywheel designs have been used in the past to address these problems.
In the first design, the peripheral speed is limited by directly winding the rotor of an external rotor type motor so that the residual displacements of the machine rotor, which is supported to a certain extent by the pre-tensioned fiber structure, will always remain below the yield point of the rotor laminations. This design results in long, cylindrical inertial bodies having a moment of inertia JT about the transverse axis of the cylinder, which is larger than the moment of inertia about the rotational axis JR of the cylinder. Because of the need for appropriately stiff mountings, this design is an unfavourable implementation from the point of view of rotor dynamics. See, Gyrostatics, breaking through the second critical rotational speed, c.f. Gasch/Pfxc3xctzner, Rotor Dynamics, Published by Springer.
In a second design, the inertial ring is separated from the electrical machine, whereby the radial displacements of the inertial ring, which now has a correspondingly larger diameter, can be considerably greater then those of the machine rotor. This can be implemented in motors using external or internal rotors since a radially elastic spoke structure is now connected merely to the rotating part of the electrical machine thereby bridging over the differences in the radial displacements. The polar moment of inertia JR in this arrangement is thereby greater than the axial moment of inertia JT, so that passing the second bending-critical rotational speed is no longer necessary. The radial elasticity of the spoke structure is frequently achieved by means of flexible spoke structures, such as those described in the U.S. Pat. No. 5,760,506 or in the dissertation by C. Wrede xe2x80x9cFlywheel mass energy store having integrated functional elementsxe2x80x9d, Dissertation TU-BS, 1998. The disadvantages inherent in such structures are the considerable axial dimensions of the electrical machine which are required at high powers because of the limited peripheral speed of the drive motor, and the necessary stiffness of the radial flexible spokes in the peripheral direction, which is required for conveying the high torque levels. Such curved spokes are generally no longer sufficiently stiff at high rotational speeds.
In the third design, the inertial body of the flywheel is arranged in a relatively thin bell-shaped shell and forms a section of a rotational ellipsoid. The shaft of a conventional electrical machine is coupled to the vertex of the ellipsoid. See, The composite fiber fly wheel as an energy store, Company publication, xe2x80x9cWTZ Rosslau GmbHxe2x80x9d, Postfach 240, 06855 Rosslau. The resilient expansions are accommodated here by the bending moments of the shell structure of the inertial ring. Although this arrangement can achieve very high specific energy densities, the absolute amount of energy that can be stored and the transferable torque are comparatively small, or it leads to dimensions for the inertial ring that are technologically not controllable as well as to problems in transferring the forces from the inertial body to the shaft.
It is an aspect of the invention to overcome one or more of the deficiencies described above.
Another aspect of the invention is to provide an arrangement in a flywheel that enables larger amounts of rotational energy to be stored as well as larger levels of torque and power to be conveyed.
In accordance with one aspect of the invention, there is provided a flywheel arrangement having an inner ring and an inertial ring that are separated by an intermediate space. This arrangement allows the tangential tensile strength of the inertial ring to be fully utilized, thereby permitting higher peripheral speeds. In addition, the separation of the inner and inertial rings increases the moment of inertia and, consequently, the stored rotational energy for a given peripheral speed. The arrangement also allows the use of inertial rings having a smaller relative thickness. A coupling device links the inner and inertial rings. Advantageously, the coupling device is radially elastic and is subjected to a radial bias force.
In another embodiment, the coupling device is formed by spring rings made of a composite fiber material.
In another embodiment, the inner ring is formed by a rotor attached to the machine, a mounting ring arranged to accept the coupling device, and a support ring made of a highly stable material.
In another embodiment, the inner and inertial rings are connected with the coupling device by an interlocking relationship.
Other aspects, features and advantages of the present invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings. The above listed aspects, features and advantages should not be construed as all-inclusive.